There would be this slight pressure for the wheel to turn one way or the other.
Relax. There are two options that will "turn your wheel for you:"
It certainly sounds like you're experiencing the "Lane Tracing Assist" function. This will literally "drive your car for you" to a limited extent, though of course you need to keep your hands on the wheel, and so on. I have tended to refer to this as "Cylon mode," as especially on the freeway, the car seems to drive to the left edge of the lane, and then to the right edge of the lane, back and forth, all the time.
The other function is the "Steering Assist" function, which will just "push the car back into the lane" if you get too close (or over) the line on the side of a lane.
I say "Relax," because you can absolutely, most definitely TURN THEM OFF.
Of course, these are all described in great detail in the owner's manual. But, that thing is so dense, I just spent about ten minutes looking for the part that tells you how to turn these thing on and off, and I still didn't get to it yet. This is a pretty good example of the "studying" you really need to do in order to understand everything that's going on with this vehicle.
So, nevermind the "owner's manual" at this point, and just go to the "Quick Reference Guide." I would suggest you go straight to the "Toyota Safety Sense" section, which gives you good descriptions of all of these "safety" functions. That starts on Page 36. This section not only goes through those systems, but also shows you the buttons -- all on your steering wheel -- that you use to both turn these things on and off, and to change settings for them. You might also have a look at the steering wheel control diagram on Page 2 of the Quick Reference Guide, which might help you visualize these buttons.
In the end, there's a button on the right side of the steering wheel that turns both of these functions on or off. And then you can go through a bunch of settings with a control on the left that lets you choose options for these functions, and many others. I would just recommend starting with turning these things off with the button on the right side, and then play with any other settings once you've had a chance to do a bit of studying. But, turning those functions off will probably take care of the issue you described, "just like that."